This invention relates to encoding a television signal, and it is also concerned with the transmission of such an encoded signal, and the reception and decoding of such a signal.
Coding of information is required when it is desired to restrict the reception of the information in intelligible form to certain recipients, for example, the subscribers to a television service.
In my earlier above-identified patent application I described several methods of encoding information. The first method includes shifting the vertical blanking pulse which results in vertical instability to a non-subscriber's television receiver. Also, video line inversion is described wherein a plurality of lines within a given field are randomly selected for inversion. In one embodiment described in the previous application, a first line for inversion is selected on a random basis for each field. Subsequent alternate lines are inverted to provide a scrambled picture.
Encoding apparatus for television signals in which the information bearing portions of some lines are inverted are shown in U.S. Pat. Nos. 3,801,732, 3,919,462, 4,022,972 and 4,025,948.